Starting to piece together my off grid solar system

01-27-2020, 06:54 PM

Hi, I’m so happy to be part of your community.

I just finished building a small cabin close to McKellar Ontario a couple of years ago and Im starting to piece together a solar system. We are only there on weekends from spring to fall and spend the occasional week there in August and September.

I was given 3 x 235 solar panels (volts: 29.8, amps: 7.9 each) and I purchased a xantrex freedom hf1000 inverter charger with built in transfer switch for ease of generator charging tie in when batteries are low and cloudy out.

I’m very new to solar and I would really appreciate your feedback. Here’s my thoughts so far:

run the 3x 235 panels in series
system volts 90
system amps 8
-run 10 wag wire 40’ from panel to charge controller
-use dc disconnect in between panels and charge controller

4 x 6V 210Ah lead acid or AGM batteries : 2 in series and then in parallel for a 12V system
- total 420 AH

on opposite battery poles then the solar charger, using 4awg cable with 100amp fuse and disconnect to the xantrex hf1000 inverter charger which connects direct to my cabin distribution breaker panel.

we have very low watt hr requirements

9 cubic ft fridge using 140 watts
lcd tv @90 watts a few hours a day
satellite receiver at 25 watts
total of 9 : 13 watt bulbs
ceiling fan: 120 watts for a few hours a day
And charging of phones here and there

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9 cubic ft fridge using 140 watts
lcd tv @90 watts a few hours a day
satellite receiver at 25 watts
total of 9 : 13 watt bulbs
ceiling fan: 120 watts for a few hours a day
And charging of phones here and there

So, I plugged this into a realistic spreadsheet and this is your modest consumption:

3x 235w = 705w derate 20% = 564 x 5hours = 2,820 watt hours max possible harvest in sunny summer day
looks like increasing your panels 4x will cover it in long bright summer days.
or reduce your loads a lot.
Thing about the mini-fridges, they skimp on insulation, and they consume as much or more as a full size efficient fridge

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i saw another reply but it’s no longer there and it was asking why 12 V and not 24V.
my xantrex inverter charger is 12 Volts and it’s less expensive to set up a 12V bank but I definitely open to suggestions

your help is really appreciated

john

I gave a quick answer but I thought Mike covered it better, and I also saw that your loads would be far too much for the proposed system, The reason for the 24V suggestion was that it would help in a couple of ways. One is that it would have allowed you to put all of the batteries you mentioned in series which is better for longevity in cycle (solar) service. The second is that it would cut the charge current and inverter battery current in half allowing for smaller wire (less amperage). But I saw that the loads you mentioned would far exceed the panel/battery combo and figured you'd be re-sizing the whole thing anyway. By the way the battery capacity needs to be about four or five times the average daily watt hour usage, so with the loads you have proposed this would be a mighty big system. Sunking put up a sticky on this very topic, let me see if I can find it. By the way welcome and glad you've joined the forum.

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I don't know much but what I do know is you don't have enough solar or battery, you need to decrease your AC reliance for running stuff especially at 12V and jump to 24v. What you have is good for RV or part of the year summer time only cabin with running generator a lot. Not trying to bust your bubble but you're out to lunch trying to live on that unless it's for a starter as you learn which is great. Lean towards buying pieces over time for system that will work.and when you got all the pieces build it and switch the flip.Takes time to learn what you can live without and learn how this functions during certain times of the year. Mike laid the numbers out that you will see will eat the system up.

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As your system grows larger, you need to increase the voltage. you can only hang so many 12V batteries in parallel before the required cabling becomes so thick and heavy, you find yourself spending hundreds of dollars in extra charge controllers (rated by amps, not volts. a 60A controller handles 720w, but at 48V, manages 2880 watts.

A 12vdc system, relying on only 12V components, is only acceptable for the very smallest hovel or RV. When running any sort of power, line losses start eating into your budget,

The other mistake made is, ignoring the AC Power Factor. Anything using simple resistance, has an ideal PF. Wind a coil, use a capacitor or transformer, and the power factor declines. Many "efficient" LED lights, have a driver circuit that has poor Power Factor. PF doesn't really increase watts consumed or heat generated, but it does place extra demands on the Power source, generator or inverter.

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just to be clear, my cabin is only a week end cabin with the odd week spent there in the summer. We don’t use it over the winter months. Also I don’t mind running the genny a few hours when needed.

my plan was to start smaller and build up over the years. I still have a bunch to do/spend at the cottage plus maintain my family at home. So buying a self sustaining system all at once isn’t realistic for me.

I need help with selecting a charge controller and sizing circuit breakers & disconnects and where to put them.

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just to be clear, my cabin is only a week end cabin with the odd week spent there in the summer. We don’t use it over the winter months. Also I don’t mind running the genny a few hours when needed.

my plan was to start smaller and build up over the years. I still have a bunch to do/spend at the cottage plus maintain my family at home. So buying a self sustaining system all at once isn’t realistic for me.

I need help with selecting a charge controller and sizing circuit breakers & disconnects and where to put them.

So here is the issue with "building up" a solar/battery system. More than likely if you start with a small system you will end up throwing away the charger, inverter and batteries because they are too small for the expansion. You end up starting all over again which can be a waste of money.

Another option would be to get a larger charger even if you don't have enough panel wattage to use it's full output. But you still have to "balance" the panel wattage and battery Ah rating or you kill off your batteries quickly by over or under charging them. Also a large inverter will use more batteries due to it's lower efficiency so it is not a good reason to go with an inverter bigger than what your battery system can provide.

The problem with a system that is very small and cheap is that you will kill off your batteries by over discharging them. So unless you only connect enough loads to use 25% discharge of your batteries each night your system fails.

Increasing your system can be done but you need to plan how much load you can connect to it and how much money you want to waste going from small equipment to larger. The same thinking needs to go with your panel wattage. Instead of buying 100watt panels it would be cheaper to get 200watt panel because of the initial $/watt cost.

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Thinking the system will recharge in the 5 days you are gone, is incorrect. The first 2 days, the batteries will be sulfating and loosing capacity, and only reaching full charge by Thursday, and then you kick the loads on and start the cycle all over again. Batteries will last the summer, and then with the shorter days in fall, it all conks out.

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i saw another reply but it’s no longer there and it was asking why 12 V and not 24V.

my xantrex inverter charger is 12 Volts and it’s less expensive to set up a 12V bank but I definitely open to suggestions

your help is really appreciated

john

As others have mentioned there are a lot of advantages to going with 24v over 12v. IMO if you already need another inverter/charger I would just go with a 48v system. If cost is an issue I think 8 6v GC2s would be cheaper and better than 4 6v AGM batteries. But at a minimum I would strongly recommend 24v over 12v especially if you already need the battery capacity.

Batteries also last significantly longer if you use less of their rated capacity.

At that, I'd say 8 * 6 = 48V system. 48V @ 200 ah = 9600 wh total, 4800 wh daily draw down. Close enough to 5kwh. Since you are weekends only, it's OK to pull batteries below 50% since you will calendar time out in 3 years before you wear them out.

You will need a 48V golf cart charger to run from your generator. 1400w of charger w/ PF of .7 needs a 2kw generator - that's a constant draw of 2kw for at least 2 hours, so a 3kw cont rated generator would work OK. Then the solar can top it off the next day after you leave.

can I use a 24V to 12v dc converter for my charger and if so what size would I need?

There is no reliable way to convert 12V @ 20A for up sizing an existing charger.
You need a new charger, a 45A MPPT charge controller and a new inverter, inverter rated at 2,000 watt cont, if you want to reliably start the fridge if you have any other loads running.

For future up-sizing, you can then simply increase the battery bank size, from 8, GC2 batteries (200ah) to 8, L-16 (400ah) (Floor Sweeper batteries) and add another 1500w or so of solar & another 45A MPPT charge controller.

If you need a silent controller , the fanless Morningstar MPPT 60 is a good one, and has a built in web server for monitoring. If you don't care about daytime fan noise, the Classic 150 is a good controller with built in ethernet too.

inverter/charger for 48v exist, when shopping pay attention to system standby losses